This invention relates generally to improvements in capacitance bridge type liquid level controls for refrigeration systems and, more particularly, to improvements in the probe which is mounted in the receiver vessel, low pressure accumulator, or other reservoir to be monitored, and to the electrical circuitry which takes capacitance readings between the probe and refrigerant holding vessel and acts in response to those readings to assure proper liquid levels of a saturated refrigerant in a pressure containing vessel.
Large compression type refrigeration systems typically used in refrigerated storage warehouses and refrigerated process cooling applications where the level of the liquid refrigerant in the low temperature accumulator must be controlled to prevent the liquid refrigerant overflowing the accumulator and passing back to the compressor suction, and to provide sufficient liquid refrigerant to handle the cooling load to the evaporator. The refrigerant liquid level is typically maintained by a solenoid valve which is cycled on and off by the liquid level control. The result is a continuous change in liquid level causing surges and splashing of the liquid refrigerant in the vessel.
Capacitance bridge type liquid level controls determine the level of a saturated refrigerant in a pressure containing accumulator vessel by inserting a metal electrically conductive probe into the liquid, insulating the probe from the liquid and the vessel, and reading the capacitance between the probe and the vessel. As the liquid level in the vessel rises and lowers the capacitance between the probe and the vessel changes. Setting and calibration of the probe-vessel capacitor allows one to determine the level of liquid in the vessel. When the capacitance bridge liquid level controls are used in closed refrigerant systems wherein a refrigerant such as freon or ammonia is used under pressure, changes in pressure, temperature and relative expansion or contraction between the probe, the electrical insulation for the metallic probe and the aperture in the vessel wall through which the probe is inserted make the sealing between these elements fraught with problems. Attempts at sealing the steel rod probe and the predominantly used annular polytetraflouroethylene (PTFE) insulation around the rod have been accomplished by annular inward pressure between the smooth outer rod surface and the smooth inner PTFE surface caused by means of tapered collars or nuts positioned annularly around the outside of the PTFE tube which are sandwiched and squeezed between threadedly connected housings. A need has arisen for improving the seal between the cylindrical rod probe and the tubular insulating PTFE material therearound.
In heretofore known electronic control circuits for capacitance bridge liquid level controls, the circuitry has been calibrated or recalibrated by disconnecting the probe from the control circuit and inserting a variable capacitor in place of the probe. After calibrating the circuit, the temporary capacitor is removed and the probe again connected into the circuit. A need has arisen for a built-in circuit in the capacitance bridge electronic control system so the unit may be tested to determine if it is correctly operating while leaving the electrical circuitry intact.
Certain heretofore known electronic circuitry for capacitance bridge liquid level controls have included transmitting circuitry positioned near the end of the probe rod to minimize the base level capacitance in the system. Other known capacitance bridge liquid level controllers have used analog meters or liquid crystal displays to display readings of the liquid level in the system. Combining the use of LED seven segment digital displays with a low cost display driver has created a need to compensate for the power supply noise created by the relatively large current draw which occurs when the digits are changed sequentially from a LED combination which draws relatively little current to a LED combination which draws substantially more current.
In heretofore known capacitance bridge liquid control systems, the effect of refrigerant splashing in the vessel from boiling action at the liquid surface, or from quick entry or exit of refrigerant from the reservoir was dampened in part by liquid level differential controls for which adjustment could be set downwardly from the initial set point of the circuitry to turn off the indicator turned on when liquid reached the level of the set point. This necessitated setting the low point reading up above the actual desired low point by the amount of the differential setting. A need has arisen to provide better circuitry to deal with violent liquid splashing in the system, and to make changes in the differential circuitry of the low point level adjustment control.
Therefore, it is an object of the invention, generally stated, to provide a new and improved probe for a capacitance bridge liquid level control system for use in refrigeration applications.
Another object of the invention is to provide improved electronic circuitry for a capacitance bridge liquid level control system for use in connection with refrigeration systems.